First make the pastry. Sift the flours, salt and xanthan gum into the bowl of a food processor and blend well. Add the margarine and blend until the mixture resembles fine breadcrumbs. Add the egg and blend for 20 to 30 seconds until the mixture comes together to form a sticky dough. There should be a little extra moisture at the base of the bowl. If it is too dry, gradually blend in one to two tablespoons of chilled water. If its too sticky, add a little rice flour.

Shape the pastry into a ball, wrap it in cling film and chill in the fridge for 30 minutes.

Meanwhile, make the white sauce by melting the margarine in a heavy-based saucepan over a low heat. Stir in the flours, then remove the pan from the heat and gradually add the chicken juices and 350 millilitres of the stock, stirring continuously. Return the pan to a medium heat and bring to the boil, stirring continuously as it thickens. Gradually add another 175 millilitres of the remaining stock. If the sauce gets lumpy, beat with a whisk until smooth.

Turn the heat down to low and simmer very gently for 10 minutes, stirring frequently to prevent the sauce from sticking to the pan. Gradually stir in the remaining stock, if necessary, to make a smooth sauce that is thick, but still runny. Season lightly with salt and pepper.

Preheat the oven to 200°C/gas mark 6. Heat the oil in a large, heavy-based saucepan over a medium heat. Add the onion and cook, stirring occasionally, for two to three minutes until just starting to turn golden, then add the leek and cook for another three to four minutes until soft.

Transfer to a two litre ovenproof casserole and stir in the chicken and tarragon. Heat the white sauce and stir it into the mixture, then season lightly with salt and pepper.

Liberally dust a chopping board with rice flour and roll out the pastry into a circle about three millimetres thick, and three centimetres wider than the casserole. Be careful as the pastry will still be slightly sticky. Ease the pastry onto the top of the casserole and cover the filling. If the dough seems too fragile to lift, simply turn the chopping board over to drop the pastry onto the casserole. Press the edges of the dough down gently to crimp and neaten the edge, using a sharp knife. Cut a small cross in the centre to let the steam out.

Bake for 40 to 45 minutes until the pastry is a rich, golden brown, then serve sprinkled with black pepper, if you like.Simply Gluten and Dairy Free

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